The use of solid particles of water insoluble organic or inorganic materials, usually referred to as "matting agents", in silver halide photographic materials is widely known in the art.
Matting agents are usually employed in the outer protective layer over the silver halide emulsion layer(s) and/or in a backing layer at the rear of the support to provide a rough surface which is often desirable in photographic art.
Matting agents are mainly used to reduce the tackiness of the photographic material, for example when the material is in the form of a roll or a stack. They can also be used to provide a sufficient roughening of the surface to prevent the formation of Newton's rings when printing and enlarging by reduction of the contact surface between two adjacent photographic materials. Further, they can be used to decrease abrasion from dry-friction and to reduce scratching when materials are stored or packed in contact with other materials as is the case, for example, of X-ray material packed without interleaves. It is also possible to use matting agents to obtain the desired coefficient of friction for film materials intended for use in automatic apparatus for rapid or super-rapid processing. Moreover, matting agents may have a slight effect in reducing the generation of static electricity in photographic elements by reducing the area of contact between the surface of the photographic element and other materials, which can cause static discharge.
The generation of static electricity can cause a large number of difficulties during the preparation and use of photographic elements, especially film elements. The generation of static electricity can be caused by rubbing of the element against rollers and other elements through or on which the materials are guided, by contact of the film element with rough surfaces or by many other known reasons. Discharges of static electricity in an unprocessed photographic element cause local film exposure and on development the discharge images become visible in the form of irregular streaks or lines and black spots. Discharges in a processed film element are also undesirable since friction can be increased.
The reduction of static electricity provided by the matting agents is insufficient, if taken alone, to solve the problem of static marks as described above.
In the art of photography, it is widely known to use specific compounds to reduce the generation of static electricity. These compounds are usually referred to as "antistatic agents". Their use, alone or in combination with matting agents can allow the production of photographic materials that do not show all of the above mentioned drawbacks.
Examples of such antistatic agents can be found, for example, in Research Disclosure, December 1989, Vol. 308, Item 308119, Section XIII and June 1982, Vol. 218, Item 21809. Research Disclosure is a publication of Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire, England. The most common antistatic agents are surface-active organic fluoro compounds as described, for example, in U.S. Pat. Nos. 3,501,653, 3,754,924, 3,775,126, 3,850,640, 3,850,642, 3,884,699, and 4,013,696 and conductive polymers as disclosed, for example, in U.S. Pat. Nos. 5,013,673, 4,668,748, and 4,810,624.
However, the most common use of matting agents is as anti-blocking agents and for this reason EP 341,200 discloses the use of monodispersed polystyrene beads, EP 118,793 discloses the use of monodispersed polymethylmethacrylate beads having an average particle size lower than 4 .mu.m, and Research Disclosure, April 1982, Item 21617 discloses the use of monodispersed polymethylmethacrylate beads having an optimal size of 1.8 .mu.m. Monodispersed matting agents are known in the art to have a better performance relative to polydispersed matting agents. The reason is probably that polymer particles having a diameter lower than 1 .mu.m are unsuitable as anti-blocking agents and can lead to a milky appearance which is undesirable when transparent images are required. Polymer particles having a diameter higher than 10 .mu.m can increase the graininess of the resulting image.
Several patents and patent applications discloses the use of monodispersed polymer particles to solve the problem of static marks. For example, U.S. Pat. No. 4,940,653 discloses the use of alkali soluble copolymers or graft polymers having an average particle size of from 0.5 to 3.0 .mu.m and U.S. Pat. No. 4,614,708 discloses the use of alkali soluble copolymers or graft copolymers having an average particle size of from 0.5 to 5.0 .mu.m. None of these patents relates to monodispersed permanent matting agents such as polymethylmethacrylate beads which are known to be difficult to prepare. U.S. Pat. No. 4,885,350 discloses a method to prepare monodispersed vinyl polymer particles, but this method is not satisfactory to prepare monodispersed polymethylmethacrylate particles having a diameter lower than 10 .mu.m. The use of radiographic materials comprising permanent matting agents in apparatus for rapid processing can cause the forming of aggregates of matting agent, removed from film by the action of rubber lips or blades, squeeze rollers or buff rollers, which can scratch the radiographic films during processing.